The Piano Scene.
The piano scene is like any other scene with a minor difference being of the sister being unclothed. The characters on the other hand behave normally like they would in everyday life.
Through that scene, Bunuel is asking his audience to rethink their basic values in life. Human beings are naturally unclothed. However society has changed the essence of humanity by requiring all of us to wear clothes. This has gone on to such an extent that the mere sight of another human in his/her elements creates confusing emotions within all of us adults.
Please consider this: Would the relationship between siblings be any different if they were in their natural states?
Its a very deep philosophical question. If humans were normally unclothed in society today, would their way of life and emotions and relationships with others be any different than what is now? Would societal and sexual norms also have been different otherwise?
What do you think?